He said: 'Today a third of women of childbearing age smoke. Doctors must warn smoking mothers of the dangers.'

The research was carried out by the Robert Debre Hospital and Pasteur Institute in Paris and the Karolinska Institute in Stockholm.

Explaining their findings, Dr Gaultier, a physiology professor at the Robert Debre Hospital, said: 'Up until the age of six months, a sleeping baby has regular short breathing pauses lasting three or four seconds which are quite normal.

'When the system is working well, the baby therefore lacks oxygen, its body moves a little bit and its brain orders it to snap awake.

'The baby increases its breathing, takes in oxygen again and goes back to sleep.

'But experiments on mice show that nicotine transmitted during pregnancy alters and diminishes the little motor which orders the waking and breathing reflex in the brain of a baby lacking oxygen.'

The researchers compared wild mice and genetically-modified mice in which beta 2 - a part of the brain where nicotine lodges - had been removed.

They injected the mice with nicotine and monitored their ability to keep breathing when deprived of oxygen.

The researchers found that the brain's reflex response to a lack of oxygen was linked to the presence of beta 2.

They went on to say that in humans, nicotine transferred from a pregnant mother to her unborn child leads to the beta 2 being constantly activated while the baby is in the womb.

This means there is a decrease in the effectiveness of the reflexes which are triggered by pauses in breathing during sleep, and therefore an increased risk of cot death.